A couple of years ago, Justice Antonin Scalia, concurring in a Supreme Court death penalty decision, took stock of the American criminal justice system and pronounced himself satisfied. The rate at which innocent people are convicted of felonies is, he said, less than three-hundredths of 1 percent — .027 percent, to be exact. That rate, he said, is acceptable. “One cannot have a system of criminal punishment without accepting the possibility that someone will be punished mistakenly,” he wrote. “That is a truism, not a revelation.” But there is reason to question Justice Scalia’s math. He had, citing the methodology of an Oregon prosecutor, divided an estimate of the number of exonerated prisoners, almost all of them in murder and rape cases, by the total of all felony convictions. “By this logic,” Samuel R. Gross, a law professor at the University of Michigan, wrote in a response to be published in this year’s Annual Review of Law and Social Science, “we could estimate the proportion o